1. Technical Field
The present disclosure relates to a semiconductor light source lighting circuit.
2. Related Art
In recent years, an LED, which has a relatively long life and low power consumption, has been used in a vehicle lamp such as a headlight instead of a halogen lamp, which includes filaments. The degree of light emission of an LED, that is, the brightness of an LED, depends on the magnitude of flowing current. Accordingly, a lighting circuit, which adjusts current flowing in an LED, is needed when the LED is used as a light source.
When a plurality of LEDs connected in series are to be lit, the decision whether to step up or step down a battery voltage in the lighting circuit is made according to the magnitude relationship between the battery voltage and the sum of forward-drop voltages of the LEDs. If dedicated lighting circuits should be designed for each of the respective cases, the variation of the lighting circuit is increased and manufacturing costs could increase.
Accordingly, a step-up/step-down DC/DC converter, which can cope with a forward drop voltage over a wide range, has been proposed (see Japanese Patent Document No. JP-A-2010-98836).
However, if the step-up/step-down DC/DC converter is used when the sum of forward drop voltages of the LEDs is higher than the battery voltage, it is disadvantageous in terms of the efficiency of divided electricity, which has a function of stepping a voltage down, as compared to a case where a step-up DC/DC converter is used.